FIG. 1 shows a typical rotary drilling rig. A drill bit 10 is connected to the end of a drill string 12. Drilling fluid, typically referred to as mud, is pumped through the drill string into the drill bit by surface equipment 11. The mud performs a variety of functions. For example, the mud cools the drill bit, cleans the cutting structures, helps penetrate the formation, and carries the cuttings to the surface. To accomplish these tasks, a high flow rate of mud must be maintained while drilling. The desired flow rate is often as high as possible based on the surface equipment 11, pressure losses in the drill string, and the capabilities of the equipment in the drill string to handle the flow.
Drill bits used to drill wellbores through earth formations generally fall within one of two broad categories of bit structures. Drill bits in the first category are known as “fixed cutter” or “drag” bits. Bits of this type usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. Drill bits of the second category are typically referred to as “roller cone” bits. An example of a prior art roller cone bit is shown in FIG. 2. The roller cone bit includes a bit body 215 having at least one roller cone 214 rotatably mounted thereto. The roller cone bit body 215 is commonly made from a plurality of legs, in this example three, that are welded together.
FIG. 3 illustrates a cross-section of one leg 301 of a prior art roller cone bit. The joining of the legs forms a fluid inlet 308 and an internal plenum 304. At least one fluid orifice 307 is typically machined in leg 301. The fluid orifice 307 comprises an entrance 302, a nozzle seat 306, an O-ring gland 310, and a receptacle 311 designed for the attachment of a nozzle 313. During drilling, fluid, not shown, enters the bit body 215 at the fluid inlet 308 and continues into the fluid plenum 304. The fluid is forced against a bottom of the fluid plenum 305 until it reaches the fluid orifice 307 where it exits the bit body 215 through the nozzle 313.
Generally, the fluid velocity within the fluid plenum 304 is relatively low. However, as the fluid moves into the fluid orifice 307, it accelerates due to the reduction of flow area. Significantly, the increased fluid velocity through the fluid orifice 307 can cause internal erosion of the drill bit. Internal erosion in a drill bit can typically be related to four parameters: mud weight, mud abrasiveness, flow velocity, and geometrical discontinuities. Over time, the drilling industry has found the need to increase the flow rates through the drill bits which has made internal erosion of the fluid orifices a significant source of concern. A ledge 314 formed between the bottom of the fluid plenum 305 and the fluid orifice 307 is particularly troublesome in drill bits. High flow rates cause the fluid flow to separate at the ledge 314 creating recirculation zones that can have sufficient energy to erode the surrounding metal surface. A “washout” occurs when the erosion progresses such that a hole is formed in the bit body 215 that allows the fluid to bypass the nozzle. The washout results in a loss of pressure in the system and requires pulling the drill bit out of the hole to be replaced. This costs the driller a great deal of time and money.
FIG. 4 illustrates one prior art solution disclosed in U.S. Pat. No. 5,538,093 (the '093 patent). In the '093 patent, a sleeve 409 is welded inside of the fluid orifice 307. The sleeve 409 comprises a smoothly contoured fluid entrance 403 which gradually reduces the flow area in preparation for entrance into a nozzle, not shown, at a nozzle seat 406. The fluid entrance 403 helps to eliminate the separation of the fluid and, therefore, reduce the amount of internal erosion. One drawback of this approach is that the sleeve 409 requires a significant amount of space to be effective. As a result, this approach is only available for the large drill bit sizes (i.e., those bits having diameters greater than 11″).
Small drill bits (i.e., those bits having diameters smaller than 11″) are typically unable to accommodate sleeves in the fluid orifices because there is not sufficient room in the interior of the bit to accommodate the required large fluid orifice without cutting into the side of the bit or into areas reserved for the bit lubrications system, not shown. FIGS. 5 and 6 illustrate a typical small drill bit. To fit a nozzle, not shown, a fluid orifice 505 is usually drilled into the bit body 508 through a bottom of the fluid plenum 504. A nozzle receptacle 509 is then formed inside the fluid orifice 505 for the attachment of the nozzle. The drilling of the fluid orifice 505 leaves a ledge 506 formed between the bottom of the fluid plenum 504 and the fluid orifice 505. Depending on the flow rate and the geometry of the particular drill bit, the ledge 506 can cause fluid separation to occur with sufficient energy to erode the bit body 508 which can lead to a washout. Manufacturing options to remove the ledge 506 are limited due to the limited space and accessibility to the ledge 506 by machining tools.
A prior art solution for small drill bits is shown in FIG. 9. A drill is inserted through the fluid orifice 505 to machine a relief region 901 substantially towards the bit body axis, not shown. While such a relief region provides some improvements in the flow, such a region fails to fully solve the erosion problems present at higher flow rates.
What is still needed, therefore, are drill bits and methods for designing and manufacturing drill bits having improved internal flow characteristics.